In the continuous digestion of cellulosic fibrous material to produce paper pulp, it is important to have effective internal circulation of liquid in order to provide for even treatment of the pulp. This is typically accomplished in continuous digesters by utilizing screen girdles at specific levels within the digester, with liquid withdrawn from the screens being treated with high pressure steam to heat it, and then recirculated to central pipes extending vertically within the digester. In order to ensure that the screens do not clog up, it is necessary to switch between the screens every few minutes (e.g. three-five minutes).
In most commercial installations, the screens are typically provided in pairs, an upper screen and a lower screen. Each screen has a header associated therewith, and typically two outlets are provided from the header, at opposite sides of the vessel around the circumference thereof. In a typical commercial installation, withdrawal is effected from the upper screen for three-five minutes, and then terminated and withdrawal is effected from the lower screen for three to five minutes and then terminated, and the sequence is repeated continuously.
The commercial installations with the above-described referencing do provide for effective treatment of the pulp, however there is one drawback associated with such a sequencing. The sequencing results in abrupt variations in the high pressure steam flow, usually the high pressure steam flow peaking when switching to the upper screen takes place, and bottoming out when switching to the lower screen takes place. This abrupt variation in steam flow may upset steam generation in certain mills.
According to the present invention, a method and simple modifications to apparatus are provided which eliminate or minimize such abrupt variations in high pressure steam flow. According to the method of the present invention there is provided a method of withdrawing liquid from first and second vertically spaced annular screens in a vertical vessel in which material is treated utilizing high pressure steam, each having an annular header associated therewith and first and second circumferentially spaced liquid outlets associated with each header, the first outlet for the first screen being generally vertically in line with the first outlet for the second screen, and the second outlet of the first screen being generally vertically in line with the second outlet of the second screen; comprising the steps of sequentially: (a) Simultaneously withdrawing liquid from the first outlet of the first screen and the second outlet of the second screen. (b) Terminating the withdrawal in step (a). (c) Simultaneously withdrawing liquid from the second outlet of the first screen and the first outlet of the second screen. (d) Terminating the withdrawal in step (c). And, (e) repeating steps (a)-(d) with an appropriate frequency and so as to minimize abrupt variations in high pressure steam flow. The method preferably comprises the further step (f) of dividing each header into two substantially equal volumes, the circumferential center of each volume being one of the outlets associated with that header, so that liquid withdrawal from the header takes place from only about one half the total volume of the header at any given time. There also may be the further step (g) of causing any material that passes through the screens to flow under the force of gravity from the circumferential ends of each header volume to the outlet associated with that header volume. Step (g) is preferably accomplished by providing downwardly sloping bottom portions from each circumferential end of each volume to the outlet associated with that volume.
According to another aspect of the present invention a particular vertical continuous digester is provided. The digester has the following conventional elements: a vertical vessel having a material inlet at the top, and a material outlet at the bottom; high pressure steam supply means for facilitating treatment of the material in the vessel; first and second vertically spaced annular screens in the vertical vessel, each having an annular header associated therewith and first and second circumferentially spaced liquid outlets associated with each header, the first outlet for the first screen being generally vertically in line with the first outlet for the second screen, and the second outlet of the first screen being generally vertically in line with the second outlet of the second screen; and pump and valve means for selectively withdrawing liquid from the header outlets. According to the present invention there is also provided: vertical wall means for dividing each header into two approximately equal volumes, an outlet associated with that header being at the approximate circumferential center of each volume; and, downwardly sloping bottom portions associated with each volume, and extending from the wall means of each volume to the outlet associated with that volume, so that any material that passes through a screen will not settle at a wall, but will move toward the outlet associated with the volume which it is associated with. The downwardly sloping bottom portions are preferably false bottoms.
It is the primary object of the present invention to provide for the effective withdrawal of liquid from a continuous digester so as to minimize abrupt variations in high pressure steam flow. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.